U.S. patent application number 17/635081 was filed with the patent office on 2022-09-15 for system and method for fabricating photonic device elements.
The applicant listed for this patent is Paul Scherrer Institut. Invention is credited to Joan Vila Comamala, Konstantins Jefimovs, Matias Kagias, Lucia Romano, Marco Stampanoni.
Application Number | 20220293427 17/635081 |
Document ID | / |
Family ID | 1000006417278 |
Filed Date | 2022-09-15 |
United States Patent
Application |
20220293427 |
Kind Code |
A1 |
Romano; Lucia ; et
al. |
September 15, 2022 |
SYSTEM AND METHOD FOR FABRICATING PHOTONIC DEVICE ELEMENTS
Abstract
Elements of photonic devices with high aspect ratio patterns are
fabricated. A stabilizing catalyst that forms a stable
metal-semiconductor alloy allows to etch a substrate in vertical
direction even at very low oxidant concentration without external
bias or magnetic field. A metal layer on the substrate reacts with
the oxidant contained in air and catalyzes the semiconductor
etching by the etchant. Air in continuous flow at the metal layer
allows to maintain constant the oxidant concentration in proximity
of the metal layer. The process can continue for a long time in
order to form very high aspect ratio structures in the order of
10,000:1. Once the etched semiconductor structure is formed, the
continuous air flow supports the reactant species diffusing through
the etched semiconductor structure to maintain a uniform etching
rate. The continuous air flow supports the diffusion of reaction
by-products to avoid poisoning of the etching reaction.
Inventors: |
Romano; Lucia; (Dottikon,
CH) ; Jefimovs; Konstantins; (Tegerfelden, CH)
; Kagias; Matias; (Zuerich, CH) ; Comamala; Joan
Vila; (Ennetbaden, CH) ; Stampanoni; Marco;
(Endingen, CH) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Paul Scherrer Institut |
Villigen PSI |
|
CH |
|
|
Family ID: |
1000006417278 |
Appl. No.: |
17/635081 |
Filed: |
July 28, 2020 |
PCT Filed: |
July 28, 2020 |
PCT NO: |
PCT/EP2020/071235 |
371 Date: |
February 14, 2022 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G02B 5/1838 20130101;
H01L 21/30604 20130101; H01L 21/308 20130101 |
International
Class: |
H01L 21/306 20060101
H01L021/306; H01L 21/308 20060101 H01L021/308 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 14, 2019 |
EP |
19191781.4 |
Claims
1-16. (canceled)
17. A method for fabricating photonic device elements by means of
metal assisted chemical etching in gas phase, the method comprising
the following steps: (a) providing a semiconductor substrate and a
patterned metal layer on the semiconductor substrate; (b) exposing
the semiconductor substrate and the patterned metal layer to
reactants in gas phase, the reactants including an oxidant gas and
an etchant gas; the oxidant gas comprising air and the etchant gas
comprising hydrofluoric acid; supplying the reactants in a
continuous or pulsed flow to the semiconductor substrate and the
patterned metal layer on the semiconductor substrate; locally
increasing a concentration of oxygen in the oxidant gas by
decomposing H.sub.2O.sub.2 on a platinum surface, being a solid
piece containing platinum immersed in a liquid solution containing
H.sub.2O.sub.2, with a decomposition of H.sub.2O.sub.2 in the
liquid phase on the platinum surface producing O.sub.2 in gas
phase; and wherein the liquid solution is placed in a container and
the liquid is not in contact with the semiconductor substrate and
the patterned metal on the semiconductor substrate.
18. The method according to claim 17, wherein the etchant comprises
hydrofluoric acid (HF) in vapor phase as evaporated from a liquid
solution containing water diluted HF.
19. The method according to claim 17, wherein the semiconductor
substrate contains a semiconductor selected from the group
consisting of: Si, Ge, or an alloy containing elements from groups
III and V in the periodic table and wherein the final metal layer
contains a metal selected from the group consisting of: Au, Ag, Pt,
Pd, Cu, Ni, Rh.
20. The method according to claim 17, which comprises heating the
semiconductor substrate and the metal patterned layer thereon to a
temperature in a range from 30.degree. C. to 90.degree. C. during
the step of exposing to the oxidant gas and the etchant.
21. The method according to claim 17, which comprises carrying out
the method in presence of an inert gas selected from the group
consisting of nitrogen, argon, and helium.
22. The method according to claim 17, which comprises carrying out
the method in the presence of an alcohol selected from the group
consisting of isopropanol, methanol, and ethanol.
23. The method according to claim 17, which comprises supplying the
etchant gas to an enclosed etching chamber by way of a dedicated
gas line.
24. The method according to claim 17, wherein the patterned metal
layer comprises a continuous mesh pattern, and the etched
semiconductor structure comprises an array of nanowires with an
aspect ratio of at least 10:1.
25. The method according to claim 17, wherein the patterned metal
layer comprises an X-ray diffractive grating pattern with periodic
features, and the etched semiconductor structure comprises an X-ray
diffractive grating with periodic features.
26. A method of fabricating photonic device elements by metal
assisted chemical etching with reactants in liquid or gas phase, to
form a semiconductor substrate and a patterned metal layer thereon,
the method comprising the steps of: (a) forming a semiconductor
oxide on the semiconductor substrate; (b) forming a plurality of
different metal layers in the patterned metal layer, wherein a
first metal layer is in contact with the semiconductor oxide of the
substrate, a final metal layer is in contact with an etching
reactant, with the first metal layer containing metals that form a
stable metal-semiconductor alloy of a compound selected from the
group consisting of silicides and germanides of one or more metals
selected from the group containing Pt, Pd, Cu, Ni and Rh; (c)
heating the substrate and the metal layer thereon in order to
realize, at the same time, a formation of the metal-semiconductor
alloy and a metal dewetting, the metal dewetting forming an
interconnected metal pattern having features, with the features of
the interconnected metal pattern being holes in the metal layer,
and the holes having a feature size of at least 1 nm.
27. The method according to claim 26, which comprises providing the
semiconductor substrate having a semiconductor selected from the
group consisting of Si, Ge, and an alloy containing elements from
groups III and V in the periodic table, and wherein the final metal
layer contains a metal selected from the group consisting of Au,
Ag, Pt, Pd, Cu, Ni, and Rh.
28. The method according to claim 26, wherein the reactant gas
comprises air.
29. The method according to claim 26, wherein the reactant gas
comprises as etching reactant HF in vapor phase as evaporated from
a liquid solution containing water diluted HF.
30. The method according to claim 26, which comprises heating the
semiconductor substrate and the metal patterned layer thereon to a
temperature in a range from 30.degree. C. to 90.degree. C. during
the step of exposing to the reactant gas.
31. The method according to claim 26, wherein the patterned metal
layer comprises a continuous mesh pattern, and the etched
semiconductor structure comprises an array of nanowires with an
aspect ratio of at least 10:1.
32. The method according to claim 26, wherein the patterned metal
layer comprises an X-ray diffractive grating pattern with periodic
features, and the etched semiconductor structure comprises an X-ray
diffractive grating with periodic features.
Description
[0001] The present invention relates to a method to fabricate high
aspect ratio patterns in a semiconductor substrate that are usable
as elements of photonic devices.
[0002] Generally, photonic devices are components for creating,
manipulating or detecting light. This can include laser diodes,
light-emitting diodes, solar and photovoltaic cells, displays and
optical amplifiers, diffractive patterns, periodic refractive and
diffractive structures, gratings and lenses.
[0003] In this context, metal-assisted chemical etching (MacEtch)
is a technique capable of fabricating 3D nano- and micro-structures
of several shapes and applications such as nanoporous layers,
nanowires, 3D objects, MEMS, microfluidic channels, Vias, X-ray
optics, sensor devices in few semiconductors--Si, Ge, poly-Si,
GaAs, SiC--and using different catalysts--Ag, Au, Pt, Pd, Cu, Ni,
Rh. In typical MacEtch, a local electrochemical etching occurs when
a metal patterned semiconductor substrate is immersed in a solution
(the electrolyte) containing an etchant (e.g. HF) and an oxidant
(e.g. H.sub.2O.sub.2). The metal serves as a catalyst for the
H.sub.2O.sub.2 reduction with a consequent holes injection deep
into the valence band of the semiconductor. The concentration of
holes becomes higher in the region surrounding the metal catalyst,
where the semiconductor is readily oxidized and removed by HF with
the formation of reaction by-products such as silicon fluoride
compounds. The reaction continues as the catalyst is pulled down
into the substrate.
[0004] The same reaction can occur when electrolyte is evaporated
and condensed on the surface of metal patterned silicon. It was
demonstrated that the MacEtch reaction occurs at room temperature
in presence of aerated HF in a similar fashion of metal corrosion
by air. The oxygen diffusion through the condensed HF/water layer
limits the etching rate and the maximum etched depth, so a maximum
depth of 6 .mu.m is etched in 3 hr. According to other sources,
etchants can be evaporated from a liquid solution containing HF and
H.sub.2O.sub.2 and adsorbed on the warmed substrate (35-60.degree.
C.), where a condensed thin layer is formed and the MacEtch
reaction occurs. MacEtch in liquid phase demonstrated the
capability to etch nanostructures with very high aspect ratio, such
as nanowires, but as a wet etching technique it suffers of bending
and agglomeration of structures during drying due to van der Waals
forces and capillary forces between adjacent surfaces at the
interface between liquid and air. The nanostructures agglomeration
is highly undesirable and considered as a limiting factor for all
the applications where the surface is directly related to the
device efficiency, such as solar cells or sensing devices.
[0005] Indeed, large bundles prevent conformal coating, deteriorate
the optical properties of an array of nanostructures, and may
induce higher series resistance. Post etching drying steps are
required to minimize the nanostructures agglomeration. For example,
CO.sub.2 based critical point drying shows excellent results, but
it still requires extra processing steps and the use of high
pressure and careful handling of the samples.
[0006] Patterning nanostructures requires high precision pattern
transferring and high lateral resolution during growing or etching,
with MacEtch in liquid this corresponds to a condition of very high
HF concentration in the etching solution. Au catalyst suffers of
bad adhesion on silicon substrates and a detrimental pattern
peel-off has been reported during MacEtch in conditions of high HF
concentration. On the other hand, uniform high aspect ratio have
been reported for nanoporous Au catalyst in conditions of low HF
concentration and high oxidant (e.g. H.sub.2O.sub.2) concentration.
In these conditions the etching is more isotropic, top of the
trenches appear wider with respect of bottom compromising the
fidelity of pattern transfer in lateral dimension, so the process
is not suitable for high aspect ratio nanostructures with high
precision of pattern transfer.
[0007] It is necessary to perform the MacEtch in conditions of low
oxidant concentration and very high HF concentration. Moreover, an
effective catalyst must be chosen to maximize the etching rate, Pt
has the faster reported etching rate for MacEtch due to its
superior catalytic activity. The use of Pt as MacEtch catalyst has
been mostly investigated in form of nanoparticles or added as top
layer of Au thick film. Pt has the advantage of forming a stable
silicide (PtSi and Pt.sub.2Si) on Si surface at relatively low
temperature, Pt silicide formation has been extensively reported in
literature for the annealing temperature in the range of
400-600.degree. C. A silicon oxide layer at the metal-substrate
interface is usually a barrier layer for metal silicide formation,
but Pt silicide has been reported to form also in presence of a
native oxide layer. The formation of a top layer of SiO.sub.2 is
possible in case of annealing in oxidizing ambient.
[0008] The use of an interconnected metal pattern has been
demonstrated to effectively reduce the off-vertical catalyst
movement during MacEtch. Thermal de-wetting of thin platinum films
offers a simple, low cost method of producing an etch mask for
fabrication of semiconductor nanowires on a large area scale.
De-wetting occurs when thin film on a solid substrate is heated,
inducing clustering of the film. The film structure morphology
(feature size, roughness, pores distribution) and the pattern
evolution strongly depend on the film/substrate parameters (film
material, film thickness, substrate material, defects) and the
experimental conditions (deposition rate, annealing temperature,
annealing environment etc).
[0009] Several research fields, such as X-ray optics, optical
devices, microfluidics and bioengineering, thermoelectric
materials, battery anodes, black silicon, solar cells, sensors and
MEMS technology can take advantage of using MacEtch as nano- and
micro-fabrication technique. In particular, MacEtch can have
applications for the fabrication of X-ray optical elements such as
gratings for grating based X-ray interferometry, zone plates,
speckles for speckle based X-ray phase contrast imaging and other
optical diffractive structures that can be used as elements of
photonics devices.
[0010] With the rise of X-ray grating interferometry access to
phase and scatter contrasts has been granted on conventional X-ray
sources, thus facilitating the potential for medical and industrial
applications. This is achieved by utilizing gratings with
micrometer sized periods, that modulate the phase or the intensity
of the X-rays. The key challenge faced at the moment is the
fabrication of such gratings in order to deliver high contrast
images over extended areas (at least 4-inch wafers). Taking into
account that the phase or intensity modulation capabilities of the
gratings are directly linked to their height/depth, for
applications operating in the medical or industrial X-ray energy
range high aspect ratios are required.
[0011] Therefore, the present invention has the objective to
provide a method to fabricate high aspect ratio patterns in a
semiconductor substrate that are elements of photonic devices, such
as diffractive gratings. Photonic devices are components for
creating, manipulating or detecting light. This can include laser
diodes, light-emitting diodes, solar and photovoltaic cells,
displays and optical amplifiers, diffractive patterns, periodic
refractive and diffractive structures, gratings and lenses.
[0012] This objective is achieved according to the present
invention by a method for fabricating photonic device elements by
means of metal assisted chemical etching in gas phase, comprising
the steps of:
[0013] (a) providing a semiconductor substrate and a patterned
metal layer thereon;
[0014] (b) exposing the semiconductor substrate and the patterned
metal layer thereon to reactants in gas phase, wherein the
reactants comprise an oxidant gas and an etchant gas, wherein the
oxidant gas comprises air and wherein the etchant gas comprises
hydrofluoric acid, and wherein the said reactants are supplied in
continuous or pulsed flow to the semiconductor substrate and the
patterned metal layer thereon, wherein the concentration of oxygen
in the said oxidant gas is locally increased by decomposing
H.sub.2O.sub.2 on a platinum surface being a solid piece containing
platinum immersed in a liquid solution containing H.sub.2O.sub.2,
wherein the decomposition of H.sub.2O.sub.2 in liquid phase on the
platinum surface produces O.sub.2 in gas phase, wherein the said
liquid solution is placed in a container and the liquid is not in
contact with the semiconductor substrate and the patterned metal
thereon.
[0015] Further, this objective is achieved according to the present
invention by a method for fabricating photonic device elements by
means of metal assisted chemical etching with reactants in liquid
or gas phase, comprising a semiconductor substrate and a patterned
metal layer thereon, wherein the semiconductor substrate and the
patterned metal layer thereon comprise the steps of:
[0016] (a) forming a semiconductor oxide on said semiconductor
substrate;
[0017] (b) forming a plurality of different metal layers in said
patterned metal layer, wherein the first metal layer is in contact
with the semiconductor oxide of the substrate, the final metal
layer is in contact with an etching reactant, wherein said first
metal layer comprises metals that form stable metal-semiconductor
alloy, wherein said metal-semiconductor alloy comprises a compound
being selected from the group consisting of silicides and
germanides of one or more metals selected from a group containing
Pt, Pd, Cu, Ni and Rh;
[0018] (c) heating said substrate and the metal layer thereon in
order to realize at the same time the formation of the
metal-semiconductor alloy and a metal dewetting, wherein the metal
dewetting comprises the formation of an interconnected metal
pattern having features, wherein the features of said
interconnected metal pattern comprises holes in the metal layer
wherein the feature size of the said holes is at least 1 nm.
[0019] Therefore, the present disclosures provide methods to
fabricate high aspect ratio patterns in a semiconductor substrate
that are elements of photonic devices, such as diffractive gratings
by using a continuous metal mesh with a stabilizing catalyst that
involves the formation of a stable metal-semiconductor alloy and
etching in presence of air in a continuous flow and an etchant. The
presence of the stabilizing catalyst allows to etch the substrate
in vertical direction even in conditions of very low oxidant
concentration (e.g. the oxidizer species being present in the air)
without any external bias or magnetic field so to realize very high
aspect ratio structures in the semiconductor substrate. Photonic
devices are components for creating, manipulating or detecting
light. This can include laser diodes, light-emitting diodes, solar
and photovoltaic cells, displays and optical amplifiers,
diffractive patterns, periodic refractive and diffractive
structures, gratings and lenses.
[0020] In a preferred embodiment of the present invention, the
patterned metal layer may comprise a bottom layer of a
metal-semiconductor alloy, wherein the said metal-semiconductor
alloy comprises a compound being selected from the group consisting
of silicides and germanides with: Pt, Pd, Cu, Ni, Rh.
[0021] Preferably, the oxidant gas can comprise air.
[0022] In a preferred embodiment of the present invention the
etchant may comprise HF in vapor phase as evaporated from a liquid
solution containing water diluted HF.
[0023] In a preferred embodiment of the present invention the
etchant may comprise a solution of water diluted HF in liquid
phase.
[0024] In a preferred embodiment of the present invention the
semiconductor substrate may contain a semiconductor selected from
the group consisting of: Si, Ge, or a III-V semiconductor and
wherein the metal may contain a metal selected from the group
consisting of: Au, Ag, Pt, Pd, Cu, Ni, Rh as a top layer
catalyst.
[0025] In a preferred embodiment of the present invention the
semiconductor substrate and the metal patterned layer thereon may
be heated to a temperature in the range from 30.degree. C. to
90.degree. C. during the exposing to the oxidant gas and the
etchant.
[0026] In a preferred embodiment of the present invention the
oxidant gas may be produced by decomposing H.sub.2O.sub.2 on a
platinum surface being a solid piece containing platinum immersed
in a liquid solution containing water diluted H.sub.2O.sub.2.
[0027] In a preferred embodiment of the present invention, the
method may be carried out in presence of an inert gas selected from
the group consisting of: nitrogen, argon and helium.
[0028] In a preferred embodiment of the present invention, the
method may be carried out in the presence of an alcohol selected
from the group consisting of: isopropanol, methanol, ethanol.
[0029] In a preferred embodiment of the present invention the
oxidant gas and the etchant gas can be connected to an enclosed
etching chamber in separated gas lines.
[0030] In a preferred embodiment of the present invention the
patterned metal layer may comprise a continuous mesh pattern, and
wherein the etched semiconductor structure may comprise an array of
nanowires with aspect ratio of at least 10:1.
[0031] In a preferred embodiment of the present invention the
patterned metal layer may comprise an X-ray diffractive grating
pattern with periodic features, and wherein the etched
semiconductor structure may comprise an X-ray diffractive grating
with periodic features.
[0032] Preferred embodiments of the present invention are
hereinafter described in more detail with reference to the attached
drawings which depict the following:
[0033] FIG. 1 is schematically showing a semiconductor substrate
covered by a multilayer metal catalyst. The semiconductor substrate
can have a thin oxide layer. The metal n.1 forms a stable
metal-semiconductor alloy with the semiconductor substrate. The
metal stuck can be composed of several metal layers.
[0034] FIG. 2 is schematically showing a Si semiconductor substrate
covered by a Pt metal catalyst layer (A) that undergoes a metal
de-wetting to form a mesh pattern and Pt silicide formation to
stabilize the catalyst (B); the etching mechanism in presence of
air and HF as etchant in gas phase (C); the formation of an etched
structure in the Si substrate (D).
[0035] FIG. 3 is schematically showing an example of apparatus to
fabricate high aspect ratio photonic devices elements with air as
oxidant, vapor HF as etchant and an open chamber in side view (A)
and top view (B). The drawing is not to scale.
[0036] FIG. 4 is schematically showing an example of apparatus to
fabricate high aspect ratio photonic devices elements with vapor HF
and O.sub.2 gas supplied by a reaction of H.sub.2O.sub.2 with a
solid Pt piece in the liquid solution containing HF, water and
H.sub.2O.sub.2, in side view (A) and top view (B). The drawing is
not to scale.
[0037] FIG. 5 is schematically showing an example of apparatus to
fabricate high aspect ratio photonic devices elements with an
enclosed etching chamber connected at least to an oxidant gas line
and an etchant gas line and eventually connected to an inert gas
line. The drawing is not to scale.
[0038] FIG. 6 shows plan scanning electron microscope (SEM) images
of Pt (bright contrast areas) film on Si (dark contrast areas)
substrate undergoing a metal de-wetting by a thermal treatment at
different temperatures.
[0039] FIG. 7 shows plan view SEM of continuous mesh patterned Pt
layer on Si (A) and cross section view SEM of etched sample after
exposure to air and HF produced by evaporation of a liquid solution
containing water diluted HF for 10 min (B) and 1 hour (C). High
magnification SEM in cross section of the formed nanowires carpet
(D) and bottom of the nanowires (E).
[0040] FIG. 8 shows the etching rate as a function of substrate
temperature (A) and HF concentration in the liquid solution (B),
the etchant evaporates from the liquid solution and the oxidant is
air. A cross section SEM (C) of etched nanowires with length of 107
.mu.m in 4 hours.
[0041] FIG. 9 shows the etching rate as a function of different
alcohols in the liquid solution (A) and at different temperatures
(B). The etchant evaporates from the liquid solution containing
water diluted HF and alcohols, the oxidant is air. Examples of
nanowires obtained in presence of Isopropanol at 40.degree. C. (C)
and 55.degree. C. (D). SEM of nanowires showing the difference in
etching depth at border of the metal pattern (E) and the reduction
of nanowires length difference as a function of temperature and
alcohol presence (F).
[0042] FIG. 10 schematically shows the process steps to realize an
high aspect ratio pattern in a semiconductor substrate: a resist
layer covers the semiconductor substrate (A); the pattern is
exposed by a lithographic method (B) and developed (C); a thin
metal layer is deposited (D) and the resist is lifted-off (E); a
continuous mesh pattern with stabilizing metal-semiconductor alloy
is formed by thermal treatment (F); the metal patterned layer is
exposed to oxidant and etchant (G) and the etched structure is
formed (H) with eventual residual nanowires.
[0043] FIG. 11 shows some examples of high aspect ratio X-ray
diffractive optical elements obtained by the present
disclosure.
[0044] FIG. 12 SEM images showing the details of the etched silicon
by MacEtch with air as oxidant (A, B) and with HF and H2O2 in
liquid phase (C, D). The typical mesoporous structure of the liquid
phase MacEtch of low resistivity silicon is highlighted by the
characteristic inverted V shape (C, D).
[0045] Photonic devices are components for creating, manipulating
or detecting light. This can include laser diodes, light-emitting
diodes, solar and photovoltaic cells, displays and optical
amplifiers, diffractive patterns, periodic refractive and
diffractive structures, gratings and lenses. The present disclosure
provides a method to fabricate high aspect ratio patterns in a
semiconductor substrate that are elements of photonic devices, such
as diffractive gratings by using a continuous metal mesh with a
stabilizing catalyst that involves the formation of a stable
metal-semiconductor alloy and etching in presence of air in a
continuous flow and an etchant. The presence of the stabilizing
catalyst allows to etch the substrate in vertical direction even in
conditions of very low oxidant concentration (e.g. the oxidizer
species being present in the air) without any external bias or
magnetic field so to realize very high aspect ratio structures in
the semiconductor substrate. The metal layer on the semiconductor
substrate reacts with the oxygen contained in the air and catalyzes
the semiconductor etching by the etchant. Air in continuous flow in
proximity of the metal layer allows to maintain constant the
oxidant concentration in proximity of the metal layer. The etchant
can be a water diluted HF solution or it can be provided by the
evaporation of hydrofluoric acid from a solution containing water
diluted HF. The continuous air flow supports the diffusion of the
reactant species (e.g. oxygen and the etchant) through the etched
semiconductor so to maintain a uniform etching rate of the high
aspect ratio structure. The continuous air flow supports the
diffusion of the reaction by-products so to avoid the poisoning of
the etching reaction. Since the oxidant gas is provided by the
normal air, the system has particular advantage for implementation
as it does not require any handling of hazardous and inflammable
gases such as O.sub.2 gas or instable chemical such as
H.sub.2O.sub.2.
[0046] The method comprises the provision of a semiconductor
substrate and a metal pattern thereon. In certain embodiments, the
semiconductor substrate can include an oxygen terminated layer or a
thin semiconductor oxide layer at the interface between the
semiconductor bulk material and the metal layer. In certain
embodiments, the metal pattern can be composed of a plurality of
different metal layers. An example of the above described
multilayer structure is reported in FIG. 1.
[0047] The first metal layer is on contact with the oxygen
terminated surface of the substrate, the final metal layer is in
contact with the etching reactants. The metals of the first layer
is chosen in the list of metals that form stable
metal-semiconductor alloy with the substrate. The metals of the
final layer is chosen in the list of MacEtch catalysts: Ag, Au, Pt,
Pd, Cu, Ni, Rh. In certain embodiments a single metal layer is
chosen, the metal is chosen in the list of: Pt, Pd, Cu, Ni, Rh. The
metal of above list can act as catalyst for MacEtch and form stable
metal-semiconductor alloy with Si and Ge as substrate, which are
called silicide and germanide, respectively. Some examples of
stable silicides that can be formed by thin film reaction are:
PtSi, Pt.sub.2Si, PdSi, Pd.sub.2Si, Pd.sub.3Si, Pd.sub.4Si,
Pd.sub.5Si, Cu.sub.3Si, NiSi, Ni.sub.2Si, Ni.sub.3Si,
Ni.sub.5Si.sub.2, Ni.sub.3Si.sub.2, Rh.sub.3Si. Some examples of
stable germanides that can be formed by thin film reaction are:
PtGe, PtGe.sub.2, PdGe, Pd.sub.2Ge, Cu.sub.3Ge, Cu.sub.5Ge.sub.2,
NiGe, Ni.sub.5Ge, RhGe, Rh.sub.2Ge, Rh.sub.3Ge, Rh.sub.5Ge.sub.3,
Rh.sub.3Ge.sub.4.
[0048] An example of the metal layer structure is reported in FIG.
1. The formation of stable metal-semiconductor alloys with Si or Ge
can be detected by XPS, TEM or RBS analyses.
[0049] In certain embodiments, the semiconductor substrate with the
metal pattern thereon is heated. During the heating, the
semiconductor substrate with the metal pattern thereon is exposed
to an oxidant gas containing O.sub.2 (e.g. air) in a continuous
flow and an acid gas containing HF such as the vapor produced by
the evaporation of a liquid solution containing water diluted HF.
The reactant gas species (gas containing O.sub.2 and HF) diffuse
through the patterned metal layer and the metal covered regions of
the semiconductor substrate are etched, thereby forming an etched
semiconductor structure. Once the etched semiconductor structure is
formed, the continuous gas flow supports the gas species diffusing
through the etched semiconductor structure. This promotes the mass
transport of the reactant species and the etching byproducts,
thereby the process can continue for long time in order to form
very high aspect ratio structures.
[0050] The presence of the stabilizing catalyst that involves the
formation of a stable metal-semiconductor alloy allows to realize a
uniform etching of the substrate in vertical direction even in
conditions of very low oxidant concentration and very dense
patterns such as the X-ray diffraction gratings.
[0051] The present method allows to reach very high etching rate in
the range of 20-24 .mu.m/hr that are comparable to values of the
liquid phase MacEtch. In reference to a previous report by Hu et
al. where a maximum depth of 6 .mu.m is reached thanks to a series
of 6 wet/dry cycles with an etching rate of 2 .mu.m/hour, with
certain embodiments of this disclosure the etching rate is improved
at least by a factor 10. With respect to a previous report by Hu et
al. where the nanowires length was limited to a maximum of 6 .mu.m
due to the limited diffusion of oxygen through the liquid etchant
layer, the present method allows to etch nanowires with at least 17
times longer length.
[0052] The method of present disclosure uses a very low oxidant
concentration, this limits the excess of charge carriers injected
in the semiconductor from the metal catalyst that is the main cause
of undesired porosity of the etched structures. Therefore, the
method of present disclosure produces almost negligible porosity
without any external bias. Moreover, the process is very stable
without any external bias or magnetic field for any pattern size
and features. With respect to a previous report by Hildreth et al.,
the presence of the stabilizing catalyst that involves the
formation of a stable metal-semiconductor alloy and the continuous
mesh pattern allow to realize uniform etching of the substrate with
uniform depth and shape of the etched structure in the vertical
direction.
[0053] Being a MacEtch reaction, the method is a promising low cost
technology for producing high aspect ratio nanostructures on large
area by surpassing the limits of other gas phase etching techniques
at the nanoscale, such as reactive ion etching. Being a gas-solid
reaction, it can be used for stiction sensitive applications
without requiring additional post etching drying processes. With
respect to previous disclosures, the method has the innovation to
use normal air as oxidant gas instead of H.sub.2O.sub.2 vapor that
comes from evaporation of a liquid solution containing water
diluted HF and H.sub.2O.sub.2. Since H.sub.2O.sub.2 is the less
volatile species in the liquid solution, it is necessary to
significantly increase the volume of H.sub.2O.sub.2 (e.g. 30%) in
the solution with respect of MacEtch in liquid phase (e.g. 1%). The
volume of H.sub.2O.sub.2 in the liquid solution limits the quantity
of HF concentration in the etchant vapor. Thus, the presence of
H.sub.2O.sub.2 in the liquid solution substantially reduces the
concentration of HF in the vapor phase. The method of the present
disclosure maximizes the concentration of HF in the etchant gas
with the advantage of extremely high precision of pattern transfer
and very high etching rate in the range of 20 .mu.m/hr. The method
has the advantage to be performed with materials that are sensitive
to the exposure with 30% H.sub.2O.sub.2, for example: cupper,
brass, carbon steel, cast iron, tungsten carbide, styrene butadiene
rubber, polysulfide polymers, thermoplastic elastomers,
thermoplastic polyurethanes, nitrile, neoprene, polyester
elastomer, and polyamides.
[0054] Moreover, the method has the advantage to avoid the handling
of heavily concentrated H.sub.2O.sub.2, while normal air is present
everywhere and free of charge. Moreover, the presence of a
continuous flow of air helps to diffuse the reactive species
through the etched substrate once a very high aspect ratio
structure is formed. The continuous flow of air through the etched
substrate promotes the supply of reactive species to the metal
catalyst allowing to continue the etching for several hours. The
continuous flow of air along the surface of the etched substrate
promotes the release and the dispersion of reaction byproduct such
as water that is detrimental for stiction sensitive nanostructures.
In certain embodiments of the present disclosure the etching is a
"dry" process, it can be used for stiction sensitive applications
without requiring additional post etching drying processes.
[0055] Described in reference to FIG. 2 is a method to fabricate
high aspect ratio patterns in semiconductor substrates, such as
diffractive gratings and other diffractive periodic structures in a
semiconductor substrate by using the metal assisted chemical
etching with a continuous flow of air and hydrofluoric acid. Then,
FIGS. 3, 4 and 5 describe some examples to realize a system for
fabricating photonic devices elements with the method of the
present disclosure.
[0056] Referring first to the flow chart of FIG. 2, the method
entails the metal layer deposition (FIG. 2A) on a semiconductor
substrate. The metal may comprise platinum (but is not limited to
platinum). The semiconductor substrate may comprise silicon (but is
not limited to silicon). The semiconductor substrate may comprise
an oxide layer (e.g. native Si oxide layer, but is not limited to
native silicon oxide). The method entails the formation of a
continuous mesh pattern of the metal layer (FIG. 2B). In certain
embodiments platinum is used as a metal layer and silicon with
native silicon oxide is used as semiconductor substrate, a
continuous mesh pattern of the metal layer is formed by thin film
de-wetting, the de-wetting temperature is about 250.degree. C. for
Pt film thickness of 10 nm.
[0057] The method entails the formation of a stable
metal-semiconductor alloy that acts as a stabilizing layer for the
metal catalyst between the metal layer and the semiconductor
substrate. In certain embodiments, platinum is used as a metal
layer and silicon with native silicon oxide is used as
semiconductor substrate, the stable metal-semiconductor alloy (e.g.
Pt silicide, PtSi, Pt.sub.2Si) is formed by annealing at the
temperature in the range of 250 to 600.degree. C. The Pt silicide
ensures a robust adhesion of the metal to the Si substrate during
MacEtch in conditions of high HF concentration. The method entails
an oxidant and an etchant. In certain embodiments the oxidant is
air and the etchant is HF. In certain embodiments the oxidant is
air and the etchant is HF evaporated from a water diluted HF
solution.
[0058] The method entails the semiconductor substrate and the
patterned metal layer thereon are exposed to air and etchant during
the heating, and air and etchant diffuse on the patterned metal
layer (FIG. 2C). In one example, silicon with a native silicon
oxide is used as semiconductor substrate, the residual silicon
oxide layer is etched away during the exposure to HF (FIG. 2C). The
metal layer acts as catalyst. The oxidant present in the air
selectively oxidizes region of the semiconductor substrate
underneath the patterned metal layer and the etchant selectively
removes the oxidized regions (FIG. 2D). Accordingly, the metal
covered regions of the semiconductor substrate are etched, inducing
the patterned metal layer to sink into the semiconductor substrate
(FIG. 2D).
[0059] Thus, an etched semiconductor structure is formed. The
etching mechanism is reported in FIG. 2D and described in detail
below. The O.sub.2 species present in the air diffuses on the
patterned metal layer, the metal acts as catalyst for the following
cathode reaction:
O.sub.2+4H.sup.++4e.sup.-.fwdarw.2H.sub.2O (1)
[0060] As a consequence, hole charge carriers are injected deep
into the valence band of the semiconductor. The concentration of
holes becomes higher in the region surrounding the metal catalyst.
Directly beneath the metal layer, the current density of holes
reaches its maximum and becomes high enough for dissolving Si there
(anode reaction). According to the literature Si can be dissolved
with two different reactions, the direct dissolution (Eq. 2):
Si+4h.sup.++4HF.fwdarw.SiF.sub.4+4H.sup.+ (2)
or via oxidation of Si (Eq. 3),
Si+2H.sub.2O+4h.sup.+.fwdarw.SiO.sub.2+4H.sup.+ (3)
followed by the dissolution of the oxide (Eq. 4):
SiO.sub.2+2HF.sub.2.sup.-+2HF.fwdarw.SiF.sub.6.sup.2-+2H.sub.2O
(4).
[0061] The reaction continues as the catalyst is pulled down into
the substrate. The etching in the gas-phase reaction takes place
via a slow gas-solid reaction. H.sub.2O is formed as by-product of
cathodic reaction (Eq. 1) and can eventually catalyze the anodic
reaction of Si oxidation (Eq.3).
[0062] FIG. 3 shows an example of system to fabricate photonic
devices elements, such as diffractive gratings. The system
comprises:
[0063] 1) flowing air as oxidant gas;
[0064] 2) evaporating HF from a liquid solution containing water
diluted HF;
[0065] 3) the semiconductor substrate with the metal pattern
thereon is placed on a heating holder;
[0066] 4) the semiconductor substrate with the metal pattern
thereon stands close to the liquid solution, that is within a few
centimeters;
[0067] 5) the semiconductor substrate with the metal pattern
thereon is heated and the etching occurs via a gas-solid reaction
being no liquid condensation formed on the sample;
[0068] 6) the holder is supported on 4 spacers on the container of
the liquid HF solution in order to form a reaction chamber that
opens pass for the air to flow in;
[0069] 7) the system is placed on a bench under laminar flow of
air.
[0070] In this example, the sample including a patterned catalyst
layer on a semiconductor substrate is supported on a hot plate or
other heating system and held within a few centimeters above the
liquid solution containing water diluted HF. The system has been
realized by modifying a simple commercial vapor HF tool, the liquid
solution was held at room temperature and the samples were held
approximately 2 cm above the liquid solution by using an
HF-compatible chuck with a resistive heating system and a substrate
temperature control.
[0071] The sample holder lays on a set of four spacers made of
teflon that are placed on the border of the container of the liquid
solution. This makes the etching chamber open and the air can
easily flow in. The system is placed on a bench in an aerated
environment under laminar flow that provides clean air. The
innovative implementation of the conventional vapor HF tool
consists in the realization of the open etching chamber by mean of
a set of four spacers between the holder and the liquid solution
container. The air flow is implemented by placing the system in air
under laminar flow, while the conventional vapor HF tool is usually
located in a fume hood with air aspiration.
[0072] In the etching system with the open chamber the air can flow
in and diffuses on the patterned metal layer and through the etched
structure. In reference to a previous report by Hu et al. where air
is only used to dry and a long series of cycling of wet/dry was
used to realize the MacEtch of silicon substrate, the innovation of
this method consists into exposing the sample to air during the
whole etching process with the advantage of a continuous etching
process. Moreover, in this method the sample is heated during the
MacEcth in order the MacEtch reaction takes place via a slow
gas/solid reaction instead of liquid/solid such as in the previous
report by Hu et al. Once the etched structure in the semiconductor
substrate is formed, the presence of air flow on the etched
structure helps also to diffuse the reactant species inside the
etched structures and to remove the reaction by-products.
[0073] The presence of air flow is relevant to etch very deep
semiconductor structures (e.g. trenches deeper than 10 .mu.m) with
very high aspect ratio (e.g. aspect ratio higher than 10:1). The
sample holder has an HF-compatible chuck with substrate temperature
control and the sample is heated to a temperature in the range from
35.degree. C. to 60.degree. C. The heating temperature has a
relevant role to avoid water condensation and nanostructures
stiction. Moreover, the etching rate of wet MacEtch is reported to
increase with temperature, therefore the efficiency of the
disclosed method is expected to increase with increasing the
reaction temperature.
[0074] FIG. 4 shows another example of system to fabricate photonic
devices elements, such as diffractive gratings with the method of
the present disclosure. In reference to FIG. 4, O.sub.2 gas is
produced in the liquid solution containing water diluted HF and
water diluted H.sub.2O.sub.2 and a solid platinum piece. The liquid
H.sub.2O.sub.2 is decomposed on the surface of the solid platinum
piece immersed in the liquid solution with the generation of
O.sub.2 gas as by-product. The O.sub.2 gas forms bubbles in the
liquid that are then exploding and releasing O.sub.2 gas. The
O.sub.2 gas can diffuse and reach the catalyst layer on top of the
sample to be etched.
[0075] The O.sub.2 gas obtained from the decomposition of
H.sub.2O.sub.2 on the platinum surface increases the O.sub.2
concentration in the air to support the MacEtch. The amount of
O.sub.2 gas released by the liquid solution can be varied by
selecting a specific volume of water diluted H.sub.2O.sub.2 to be
present in the liquid solution containing the water diluted HF and
the water diluted H.sub.2O.sub.2. The amount of O.sub.2 gas
released by the liquid solution can be varied by selecting a
specific area of the solid platinum piece to be immersed in the
liquid solution containing the water diluted HF and the water
diluted H.sub.2O.sub.2. The uniformity of the O.sub.2 gas released
by the liquid solution can be varied by selecting a specific shape
(e.g. a platinum wire mesh) of the solid platinum piece to be
immersed in the liquid solution containing the water diluted HF and
the water diluted H.sub.2O.sub.2. This embodiment of the method
allows to supply the concentration of O.sub.2 gas in the air by
keeping the etching chamber closed. With respect to a previous
report by Hildreth et al., the method of the present disclosure
allows to obtain higher etching rate since the concentration of
oxidant is increased with respect to the concentration of
evaporated H.sub.2O.sub.2.
[0076] FIG. 5 shows another example of system to fabricate photonic
devices elements with the method of the present disclosure. The
example contains at least two separated and independent gas lines,
at least one gas line for an oxidant gas and at least one gas line
for an etchant gas, being each gas line in fluid connection to an
etching chamber. An additional gas line can provide a non-reactive
gas as purging (e.g. nitrogen or argon). The semiconductor
substrate and the metal pattern with the stable metal-semiconductor
alloy thereon are placed on a sample holder, the holder lays in an
enclosed etching chamber, the etching chamber can be eventually
evacuated. The sample holder can eventually provide the sample
heating. The gas flow in each gas line in fluid connection to the
etching chamber can be independently regulated. The sample can be
exposed to the oxidant and the etchant gases by flowing both gases
at the same time or by flowing one gas per time with an eventual
step of purging gas and an eventual step of chamber evacuation. The
oxidant and the etchant gas can flow and diffuse on the metal
pattern and thereby forming an etched semiconductor structure. The
sample can eventually be heated during the exposing of the oxidant
and etchant gases.
[0077] The proposed etching tool differs from the one by Hu et al.
since the present method does not flow oxygen gas through a liquid
HF solution. The innovation here disclosed is characterized by the
presence of separated gas lines for oxidant and etchant. In
particular, in the present invention, the etchant gas can be
anhydrous HF and the semiconductor substrate with metal pattern
thereon is heated during the exposure to the etchant atmosphere in
order to minimize the presence of water, being water condensation
detrimental for producing high aspect ratio nanostructures.
[0078] Using a catalyst that has high efficiency reaction with
oxidizers, such as platinum, the method of the present disclosure
can etch the semiconductor substrate for several hours in a gas
atmosphere that contains a very small amount of oxidant and a high
concentration of etchant, producing very deep trench (e.g. 100
.mu.m), huge aspect ratio structures (in the range of 1000-10000 to
1) and very sharp features at the scale of 1 to 100 nm. In certain
embodiments, a self-assembled platinum metal pattern on top of a
silicon substrate is used to produce a carpet of high aspect ratio
silicon nanowires. In certain embodiments, a thermal treatment is
used to induce the platinum film de-wetting with the consequent
formation of a nanostructured metal pattern. De-wetting occurred
for Pt deposition on oxygen terminated Si surface, whilst no
de-wetting was observed under the same experimental conditions when
the native oxide was removed by dipping the substrate in HF
immediately before the Pt deposition.
[0079] Described in reference to FIGS. 6 is an example of tuning
the size distribution of holes produced by de-wetting of thin Pt
film on Si substrate with native silicon oxide layer. In this
example, the Si substrate with native silicon oxide was cleaned by
oxygen plasma, then a Pt film was deposited by electron beam
evaporation with a deposition rate of 0.5 nm/min and Pt film
thickness in the range of 5 to 20 nm. The substrate with the metal
film thereon was annealed in air at temperature in the range of
250.degree. C. to 600.degree. C. to produce the metal film
de-wetting. FIGS. 6A-I shows the SEM images of Pt film morphology
at different de-wetting temperature. Referring to FIGS. 6A-I, the
metal has bright contrast while the holes show the silicon
substrate in a darker grey. Thus, the metal layer is patterned in a
self-assembly nanostructure, the metal holes have size distribution
in the range of few nanometers to hundredth nanometers.
[0080] Thus, the perforated Pt film of FIGS. 6A-I are examples of
self-assembled metal mask for the realization of nanowires by
MacEtch. The de-wetting occurs with a progressive increase of film
fractures density (250-350.degree. C.) and finally the hole
formation appeared (400-500.degree. C.), followed by a coalescence
process of holes expansion (550-600.degree. C.). Once the film
thickness and deposition conditions are fixed, the de-wetting
temperature can be used as a tuning parameter for the features size
of the Pt pattern, the average hole size increases from few
(<400.degree. C.) to tens (450-550.degree. C.) and hundreds
(>550.degree. C.) of nanometers. Pt silicide formation has been
extensively reported in literature for the annealing temperature in
the range of 400-600.degree. C.
[0081] A silicon oxide layer at the metal-substrate interface is
usually a barrier layer for metal silicide formation, but Pt
silicide has been reported to form also in presence of a native
oxide layer. The formation of a top layer of SiO.sub.2 is possible
in case of annealing in oxidizing ambient. The growth of asymmetric
holes during de-wetting is observed in all FIGS. 6 and it is an
indication of silicide formation.
[0082] Described with reference to FIG. 7 is the realization of
nanowires by the method of the present disclosure with Pt
self-assembled metal mask by de-wetting. Nanowires can be used as
diffractive optics in speckle based X-ray phase contrast imaging.
Nanowires are expected to improve the sensitivity of speckle-based
X-ray imaging by producing speckles of smaller size and much better
uniformity in comparison to sandpaper or other membranes with
feature size in the micron range.
[0083] A thin Pt film was deposited on Si substrate with native
silicon oxide layer, the substrate with the metal film thereon was
annealed in air at 550.degree. C. to produce the metal film
de-wetting. A scanning electron microscope (SEM) micrograph in plan
view is reported in FIG. 7A. The substrate with the metal pattern
thereon is heated at 55.degree. C. for 10 min and then exposed to a
gas phase etchant. The gas phase etchant is produced by using the
system of FIG. 3. The oxidant is provided by flowing air. The
etchant is evaporated from a liquid solution containing water
diluted HF, the HF concentration in the liquid is in the range of 1
to 20 mol/l. The substrate with the metal pattern is held at 2 cm
from the liquid surface. The gas phase etchant diffuses through the
metal pattern, the silicon substrate behind the metal is etched and
the metal pattern sinks into the substrate. Thus, a silicon etched
structure is formed. After 10 min of etchant exposure the substrate
is clearly etched and the silicon etched structure appear like
pillars, as shown in the SEM micrograph in cross section of FIG.
7B.
[0084] The metal mask of FIG. 7A and the silicon pillars of FIG. 7B
show a good matching of the structural features, indicating that
the method has excellent capability of pattern transfer at the
nanometer scale.
[0085] FIG. 7C shows the silicon structure after one hour exposure
to the etchant, the silicon pillars are now 6 .mu.m long and can be
called nanowires. A uniform carpet of silicon nanowires (FIG. 7C)
is formed by the disclosed method of the present invention. FIG. 7D
shows a magnified image of the silicon nanowires, the top nanowires
are well separated with reduced agglomeration with respect of
nanowires produced by wet etching methods. FIG. 7E shows a
magnified image of the bottom of the silicon nanowires, the
nanowires section is measured by SEM and is in the range of 10 to
100 nm. The aspect ratio of the nanowires is calculated by the
ratio of the average section diameter (e.g. 10 to 100 nm) and the
nanowires length (e.g. 6 .mu.m). Thus, the aspect ratio is in the
range of 60 to 600 to 1.
[0086] FIG. 8A reports the etching rate for the system showed in
FIG. 3 as a function of the heating temperature of the silicon
substrate and the metal pattern thereon and the molar concentration
of HF in the liquid solution containing water diluted HF. The
etching rate has been calculated by measuring the length of
nanowires produced in 2 hours in the experimental set up of FIG. 3.
The samples are square of 1.times.1 cm.sup.2 cleaved from a silicon
substrate with platinum self-assembled mask by de-wetting, the
silicon substrate is N type <100> single crystal with
resistivity in the range of 0.001 to 0.01 .OMEGA.cm. The liquid
solution has been obtained by adding deionized water to a
commercial water diluted HF solution at 50%. By increasing the
temperature in the range of 35 to 40.degree. C., the etching rate
increases in agreement with previous studies on MacEtch kinetics in
liquid. For high HF concentration (18 mol/l), the etching rate has
a clear maximum at 40.degree. C., then it decreases as a function
of temperature, indicating that the reaction rate is limited by the
HF desorption. For the low HF concentration (12 mol/l), the etching
rate slightly increases with the temperature but the variation of
etching rate is quite small (15%) in the full range of temperature
(35 to 55.degree. C.). Indeed, this represents a remarkable stable
processing window, where the degradation of HF concentration with
time can have a negligible effect on the etching rate.
[0087] FIG. 8B shows the etching rate for the system showed in FIG.
3 at 55.degree. C. as a function of the molar concentration of HF
in the liquid solution containing water diluted HF. The liquid
solution is obtained by adding deionized water to a commercial
water diluted HF solution at 50%. Very high etching rate of 20 to
24 .mu.m/h are reported for very high HF concentration, these
values are comparable to MacEtch in liquid phase. In reference to a
previous report by Hu et al. where a maximum depth of 6 .mu.m is
reached thanks to a series of six wet/dry cycles with an etching
rate of 2 .mu.m/hour, with the method of the present disclosure the
etching rate is improved at least by one order of magnitude.
[0088] FIG. 8C shows the SEM in cross section of silicon nanowires
with a length of at least 107 .mu.m obtained by heating the silicon
with platinum mask thereon at 55.degree. C. during the exposition
to air and HF evaporated from a liquid solution with molar
concentration of HF in the range of 20 to 29 mol/l for 4 hours.
Being the nanowires section in the range 10 to 100 nm, the aspect
ratio of the nanostructures in FIG. 8B is in the range of 1000 to
10000 to 1. The nanowires have low agglomeration, indicating that
the reaction was happening in the gas phase during the whole time.
With respect to a previous report by Hu et al. where the nanowires
length was limited to a maximum of 6 .mu.m due to the limited
diffusion of oxygen through the liquid etchant layer, the method of
the present disclosure allows to etch nanowires with at least 17
times longer length. The Pt catalyst layer is still visible at the
bottom of the SEM image of FIG. 8C, it looks flat indicating that
it is still stable even after the long etching and exposure to
heavily concentrated HF gas.
[0089] The stability of the catalyst indicates that the gas phase
MacEtch can continue and produce even longer nanowires. Thus, FIG.
8 demonstrates the capability of the present invention to etch
extremely deep trenches with huge aspect ratio (10000 to 1) in
silicon with very high precision.
[0090] In another example, the etchant is obtained by evaporation
of a liquid solution that contains water diluted HF and alcohol as
additive. Alcohols with low vapor pressure and low surface tension
is used as catalyst instead of water vapor in order to minimize the
capillary force of the gas-liquid interface. The alcohol helps the
vapor etching to proceed with smaller water condensation because it
is highly volatile and tends to evaporate easily with water.
[0091] FIG. 9 reports the experimental results obtained with the
setup of FIG. 3 and the use of methanol, isopropanol and ethanol as
additive in the liquid solution containing water diluted HF. The
samples are square of 1.times.1 cm.sup.2 cleaved from a silicon
substrate with platinum self-assembled mask of FIG. 6, the silicon
substrate is N type <100> single crystal with resistivity in
the range of 0.001 to 0.01 .OMEGA.cm. The liquid solution has an HF
molar concentration in the range of 1 to 20 mol/l, the alcohol
volume is in the range of 10 to 20% of the full liquid
solution.
[0092] FIG. 9A shows the etching rate calculated by measuring the
length of the nanowires produced at 40.degree. C. in 2 hours with
the system of FIG. 3. The etching rate decreases in presence of
alcohols as reported in FIG. 9A. In this particular example, the
etching rate has the highest value for isopropanol. FIG. 9B reports
the SEM image of nanowires produced by adding isopropanol to the
liquid solution containing water diluted HF and heating the
substrate with the metal pattern thereon at 40.degree. C. The
liquid condensation causes the nanowires to form large bundles. The
alcohol catalyzes the HF reaction by producing water as by-product
so the thickness of the condensed layer increases with the alcohol
content in the vapor.
[0093] The etching rate decreases as a function of the substrate
temperature as reported in FIG. 9B. By rising the temperature up to
55.degree. C., no nanowires bundles were detected.
[0094] FIG. 9D reports the SEM image of nanowires produced by
adding isopropanol to the liquid solution containing water diluted
HF and heating the substrate with the metal pattern thereon at
55.degree. C. The nanowires of FIG. 9D have the same length of
nanowires in FIG. 9C but they appear well separated. Thus, the
heating temperature is a relevant parameter to avoid water
condensation and nanostructures stiction in the method of the
present disclosure. The etching proceeds with higher etching rate
at the border of the pattern, an example of this effect is visible
in FIG. 9E.
[0095] FIG. 9F shows the relative variation of the length
(.DELTA.L) of the nanowires between the center and the border of
the sample as a function of the substrate temperature, the higher
the temperature the smaller the .DELTA.L. In presence of alcohol
the reduction of .DELTA.L is even more relevant. Therefore, the
etching uniformity can be improved by increasing the heating
temperature and in presence of alcohol.
[0096] FIG. 10 shows a flow chart for fabricating photonic devices
elements, such as diffractive gratings with the method of the
present disclosure. In certain embodiments, the metal pattern is
prepared by lithographic methods with positive resist on a silicon
substrate. A proper resist coats the semiconductor substrate (FIG.
10A), the resist type and the thickness depend on the desired
pattern.
[0097] In one example a positive photoresist MICROPOSIT.TM. S1805
was used for photolithography, according to a procedure reported
elsewhere. In another example PMMA as positive resist was used for
electron beam lithography. The resist is exposed to UV or e-beam
lithography (FIG. 10B) and subsequently developed (FIG. 10C). A
short plasma cleaning (10 to 60 s in a standard oxygen RF plasma
etching) was used to clean the resist residual, the time was tuned
to avoid an excessive thinning of the resist. In certain embodiment
Pt was used as metal catalyst, Pt was deposited using an electron
beam evaporator with a deposition rate of 0.5 nm/min. The Pt
thickness was the range of 5 to 20 nm (FIG. 10D). Then, lift-off
was performed (FIG. 10E), for example by dipping the sample in
acetone. The sample was rinsed in clean solvent, then in
isopropanol and dried by nitrogen blowing. The Pt film de-wetting
and the Pt-silicide to stabilize the catalyst layer is obtained by
annealing (FIG. 10F) on a hot plate in air in the temperature range
of 250-600.degree. C. The de-wetting step is relevant to ensure a
uniform etching of metal patterns that have feature size in one
direction bigger than 500 nm.
[0098] The metal de-wetting produces nanowires during MacEtch. The
impact of etched nanowires on the final pattern can be minimized by
tuning the metal film thickness and the annealing temperature in
order to have nanowires with section size much smaller than the
pattern feature size, such as in the examples of FIG. 6. In certain
embodiments the MacEtch is performed by exposing the Si substrate
and the Pt patterned layer with the stabilizing Pt silicide layer
thereon to air and HF during the heating (FIG. 10G). The metal
layer acts as catalyst. The oxidant selectively oxides region of
the semiconductor substrate underneath the patterned metal layer
and HF selectively removes the oxidized regions (FIG. 10H).
[0099] Accordingly, the metal covered regions of the semiconductor
substrate are etched, inducing the patterned metal layer to sink
into the semiconductor substrate (FIG. 10H). Thus, an etched
semiconductor structure is formed.
[0100] FIG. 11 shows some examples of gratings structures obtained
by the procedure showed in FIG. 10. The metal layer was patterned
by UV photolithography for the examples in FIG. 11A and by electron
beam lithography for the examples in FIGS. 11B-D. FIG. 11A shows a
linear grating with pitch size of 4.8 .mu.m. The nanowires produced
by the Pt de-wetting are visible in the SEM image but they have
minimal X-ray absorption. FIG. 11B shows a circular grating with
pitch size of 1 .mu.m. Residuals of nanowires are visible in the Si
trench due to the catalyst de-wetting. A depth of 29 .mu.m was
realized by heating the sample at 55.degree. C. and exposing for 4
h in the system described in FIG. 3. The resulting aspect ratio is
about 80:1. The smoothness of the etched Si lines is visible in the
high resolution images of the grating from top (FIG. 11C) and
bottom (FIG. 11D) views. The etching is very uniform on the whole
patterned area as demonstrated by the uniform Moire pattern visible
in the SEM image (FIG. 11B).
[0101] FIG. 12 shows an example of linear grating with pitch size
of 1 .mu.m and silicon width of 300 nm, the metal pattern was
produced by electron beam exposure of PMMA resist and Pt
deposition. FIGS. 12A and 12B (B is high magnification detail of A)
is a cross section SEM of the bottom of the etched structure by the
method of present disclosure as described in FIG. 3. The etching
was realized at 55.degree. C. with air and etchant produced by
evaporation of a liquid solution containing water diluted HF at a
molar concentration in the range of 1-20 mol/l.
[0102] FIGS. 12C and 12D (D is high magnification detail of C) is a
cross section SEM of the bottom of the etched structure by liquid
phase MacEtch, the liquid solution contains water diluted HF in a
molar concentration of in the range of 1-5 mol/l and H.sub.2O.sub.2
in a molar concentration of 0.5-2 mol/l. FIG. 12 is meant to show
the Si porosity of structures realized by MacEtch in gas phase in
comparison to liquid phase for Si N type <100> single crystal
with resistivity in the range of 0.001-0.01 .OMEGA.cm. In gas phase
(FIG. 12A-B) the etched Si structure has the same contrast of bulk
Si (below the catalyst), few nanowires are visible on the catalyst
layer. A plurality of small pores are visible by SEM in the
structure produced by liquid phase MacEtch (FIG. 12C-D). Moreover,
a characteristic distribution of pores is observed at the bottom of
the etched structures (FIG. 12C-D), the pores are so many that they
look like a full mesoporous structure, the non-etched bulk Si is
clearly visible at the bottom of the image. The mesoporous Si has a
poorer contrast in SEM in comparison to bulk non-etched Si regions.
The mesoporous structure starts at the interface with non-etched
bulk Si substrate with a characteristic inverted V shape in
proximity of the Pt catalyst layer.
RELEVANT PRIOR ART
[0103] Y. Hu, K.-Q. Peng, Z. Qiao, X. Huang, F.-Q. Zhang, R.-N.
Sun, X.-M. Meng & S.-T. Lee, Metal-Catalyzed Electroless
Etching of Silicon in Aerated HF/H2O Vapor for Facile Fabrication
of Silicon Nanostructures, Nano Letters 14 (2014) 4212-4219.
[0104] O. J. Hildreth & D. R. Schmidt, Vapor Phase
Metal-Assisted Chemical Etching of Silicon, Advanced Functional
Materials 24 (2014) 3827-3833.
[0105] Catalyst assisted chemical etching with a vapor phase
etchant according to US 2018/0090336 A1.
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